21. The model with epicycles is in fact a very good model of an elliptical orbit with low eccentricity. 22. Newtonian or classical mechanics eliminated the need for deferent / epicycle methods altogether and produced more accurate theories. 23. Only after Kepler's refinement of Copernicus'theory was the need for deferents and epicycles abolished. 24. Shen's hypotheses were similar to the concept of the epicycle in the Greco-Roman tradition, 25. Most historians of astronomy consider that this two-epicycle model reflects elements of pre-Ptolemaic Greek astronomy. 26. At around 500 BC, the Greeks invented the idea of epicycles , of circles traveling on the circular orbits. 27. When scholars applied Ptolemy's epicycles , they presumed that each planetary sphere was exactly thick enough to accommodate them. 28. Although Copernicus'models reduced the magnitude of the epicycles considerably, whether they were simpler than Ptolemy's is moot. 29. Ptolemaic model of the spheres for Venus, Mars, Jupiter, and Saturn with epicycle , eccentric deferent and equant point. 30. In models of the universe that precede Ptolemy, generally attributed to Hipparchus, the eccentric and epicycles were already a feature.
